Profiled strips extruded of plastic materials are widely used in automobile body applications as protective molding pieces. Molding pieces such as body side moldings or wheel well moldings are used to protect a vehicle body from impact damage and to protect edges of sheet metal panels from corrosion. Most of these molding pieces are extruded of thermoplastic materials such as PVC (poly vinyl chloride), thermoplastic rubber, thermoplastic urethane, or other flexible plastic materials. PVC is one of the most widely used materials for its low cost and good physical properties.
Molding pieces such as a body side molding may be extruded of a plastic material alone or extruded in an extrusion coating process where plastic material is extruded on a metal reinforcing substrate. In an extrusion coating process where plastic material is combined with a more rigid metal substrate, a cross-head extrusion technique is used. In a cross-head extrusion process, a continuous length of metal reinforcing substrate is pulled through an extrusion die at 90.degree. angle to the direction of the plastic flow when molten plastic material is being extruded onto the metal substrate. The metal reinforcing substrate is usually cleaned and dried through a multi-station surface treatment process and formed by a series of metal roll formers prior to the entry into the extrusion die.
In automobile body parts, the aesthetic appearance of the part is one of the most important criteria that must be satisfied. In the extrusion of vinyl or vinyl/metal substrate reinforced molding pieces, the quality of the surface appearance of the molding pieces obtained is dependent upon many variables. For instance, these variables include the material composition, the extrusion temperature, the extrusion pressure, the extrusion rate, the cooling rate, etc. Minor irregularities in the extrusion conditions can cause surface imperfections in the molding pieces such as shock lines, shock marks, or ripples. They normally show up as lines or ripples across the extrudate 90.degree. to the extrusion direction. These unsightly lines destroy the aesthetic appearance of the extrudate and frequently result in the scrap of the molding pieces.
It is, therefore, an object of the present invention to provide a method of extrusion of profiled extrudates of plastic material having a desirable smooth surface finish.
It is another object of the present invention to produce profiled extrudate of polymeric material that has substantially no surface defects of shock lines, shock marks or ripples.
It is a further object of the present invention to produce profiled extrudate of plastic material having smooth aesthetic surface finish achievable by a simple process modification.